Stimulation of erythrocyte phosphatidylserine exposure by lead ions

Pb+ intoxication causes anemia that is partially due to a decreased life span of circulating erythrocytes. As shown recently, a Ca2+-sensitive erythrocyte scramblase is activated by osmotic shock, oxidative stress, and/or energy depletion, leading to exposure of phosphatidylserine at the erythrocyte surface. Because macrophages are equipped with phosphatidylserine receptors, they bind, engulf, and degrade phosphatidylserine-exposing cells. The present experiments were performed to explore whether Pb+ ions trigger phosphatidylserine exposure of erythrocytes. The phosphatidylserine exposure was estimated on the basis of annexin binding as determined using fluorescence-activated cell sorting (FACS) analysis. Exposure to Pb+ ions [ greater than or equal to 0.1 muM Pb(NO3)(2)] significantly increased annexin binding. This effect was paralleled by erythrocyte shrinkage, which was apparent on the basis of the decrease in forward scatter in FACS analysis. The effect of Pb+ ions on cell volume was virtually abolished, and the effect of Pb+ ions on annexin binding was blunted after increase of extracellular K+ concentration. Moreover, both effects of Pb+ ions were partially prevented in the presence of clotrimazole ( 10 muM), an inhibitor of the Ca2+-sensitive K+ channels in the erythrocyte cell membrane. Whole cell patch-clamp experiments disclosed a significant activation of a K+-selective conductance after Pb+ ion exposure, an effect requiring higher ( 10 muM) concentrations, however. In conclusion, Pb+ ions activate erythrocyte K+ channels, leading to erythrocyte shrinkage, and also activate the erythrocyte scramblase, leading to phosphatidylserine exposure. The effect could well contribute to the reported decreased life span of circulating erythrocytes during Pb+ intoxication.